WASHINGTON (CNN) --After days of impassioned debate, the U.S. Senate Tuesday approved a $400 billion plan to overhaul Medicare. Supporters say it will give prescription drug coverage to 40 million older Americans, while critics warn that it could destroy the system.

The 54 to 44 vote was not along party lines -- 11 Democrats voted in favor and nine Republicans voted no.

The bill now goes to the White House, for President Bush's signature.

At a speech Tuesday at a hospital in Las Vegas, Nevada, Bush called the vote a "major victory" and thanked members of Congress for their hard work.

"We inherited a good medicare system. It has worked, but it was becoming old and needed help. Because of the actions of the Congress, because of the actions of members of both political parties, the medicare system will be modern and it will be strong," Bush said.

Supporters of the bill -- including some Democrats who, like Sen. Dianne Feinstein of California, said it was better than no bill at all -- hailed the vote as a victory for senior citizens.

"This bill is an extraordinary day for seniors and indeed all Americans," said Senate Majority Leader Bill Frist. "The legislation we just passed is consequential, it is far reaching. ... It is epochal in the sense it modernizes Medicare to provide 21st century care for our seniors."

Opponents, however, vowed that the fight isn't over. Senate Minority Leader Tom Daschle predicted that once seniors learned how the bill worked, they would mobilize as they did 38 years ago for the original Medicare bill.

"I was struck by how vacant the galleries were and so few seniors citizens looking down," Daschle, D-South Dakota, said after the vote. "What you saw instead were lobbyists packing the halls. They will do well. Our seniors will not, and that is why the fight will go on."

Drug benefit starts in 2006

The measure, a centerpiece of President Bush's domestic agenda, adds a prescription drug benefit to the program, provides billions of dollars in subsidies to insurance companies and HMOs, and takes the first step in allowing private plans to compete with Medicare.

It is the largest expansion of Medicare since it was created in 1965, although most of its provisions won't take effect for several years. The drug benefit, for example, does not take effect until 2006. Before that, seniors will be able to purchase, within six months, a discount card that could provide 10 to 25 percent off prescription drugs. (Interactive: Prescription for change)

Supporters say the bill helps lead to better private coverage for seniors. But opponents say it wastes taxpayer funds and effectively forces seniors into inadequate and expensive health plans.

Opponents also warned that seniors would demand that Congress revisit the issue once they realized what the bill does and does not do. High on the list of things not covered in the bill is some mechanism to stem the rising costs of prescription drugs. Even Feinstein, who supported the bill, called that a "major weakness in this bill."

"The theory is that private sector competition will drive down the cost of drugs," she said. "That may happen, or it may not happen. We need to watch that, and we will. I feel confident that the leadership will make changes if the cost containment is not kept."

Sen. Ted Kennedy, D-Massachusetts, who has long fought for a prescription benefit but vehemently opposed this bill, promised to take the issue "to the highways and byways of this country and to the senior citizen centers and nursing homes where the senior citizens gather."

"We will continue this battle here in the United States Senate and in the course of the elections," Kennedy said. "I'm absolutely sure that at the end of the day we will preserve the Medicare system ... and we will get to the day when we have a real prescription drug program."

Sen. Barbara Mikulski, D-Maryland, said senators had "squandered the opportunity to truly change history and to truly change the lives of senior citizens" by providing them with "a skimpy benefit."

"This issue is not finished," she said. "I intend to come back in January ... to really try to get control of the cost of prescription drugs and to try to redirect these enormous subsidies that are going to insurance companies."

Despite the strong Democratic opposition to the bill, Frist, R-Tennessee, praised it as a bipartisan effort.

"It was bipartisan legislation from day one," he said. "It is with great satisfaction that we stand together in a bipartisan way with a bipartisan vote."

Among the 11 Democrats who voted for the bill was Sen. John Breaux, D-Louisiana, who said it was "a balanced piece of legislation that is truly reform."

"This is the end of a Medicare program that forces seniors to choose between food on the table and the medicine that they need," he said.

Fellow Democrat Max Baucus of Montana said he was "extremely proud" of the bill, which he said brings "Medicare up into this century."

Jim Jeffords of Vermont, the former Republican who turned Independent, also voted for the bill.

Kerry, Lieberman skip vote

The Senate voted 54 to 44 in favor of the Medicare bill on Tuesday.

The Democratic opponents of the bill were joined by nine Republicans who objected to the bill as an expensive entitlement program.

Two key Democrats did not vote. Presidential candidates John Kerry, D-Massachusetts, and Joe Lieberman, D-Connecticut, participated in Monday's debate but skipped Tuesday's vote.

Lieberman's press secretary, Jano Cabrera, said the senator left for a campaign stop in Arizona -- where Bush is to appear at a fund-raiser Tuesday afternoon for Rep. Rick Renzi's re-election campaign -- after it became "ultimately clear the bill was heading toward passage."

Cabrera said Lieberman would "continue to speak out against the president's Medicare policies."

In a written statement, Kerry said he fought "tooth and nail against this special interest giveaway" but returned to the campaign trail once he decided that his vote would not make a difference.

Prior to the vote, senators on both sides of the issue made impassioned pleas for their side. But two last-minute efforts by Democratic senators failed to block the vote, and the bill won out.

The House passed the bill in a controversial vote early Saturday after late-night phone calls from the president, a move credited with helping get the bill passed there. A three-hour vote was ended by GOP leaders at 6 a.m., after a 218 to 216 deficit flipped to a 220 to 215 victory.